School of Agriculture, Food and Ecosystem Sciences - Theses

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Type: PhD thesis × Start date: 2000 × Author: Crowfoot, Lisa Victoria ×

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    Phenotypic variation in raphanus Raphanistrum L
    Crowfoot, Lisa Victoria (University of Melbourne, 2005)
    Raphanus raphanistrum is a very successful weed throughout the world. In Australia, it is particularly a problem within the southern wheat belt. This region includes a wide range of climates and different management practices. It is likely that the extensive phenotypic variation seen in R raphanistrum has contributed to its success in these environments. This thesis set out to determine the importance of the different sources of phenotypic variation (genetic variation, phenotypic plasticity and, within these, maternal effects) on growth and development of Raphanus raphanistrum. It examines these factors individually and, using the concept of the integrated (multivariate) phenotype, the ways in which they combine to form such a successful plant. The thesis reports on a sequence of field experiments using nitrogen and competition gradients to determine the effect of genetic variation, phenotypic plasticity, seed size and their interactions on growth and development. Furthermore, to gain an understanding of how genetic variation and phenotypic plasticity can both be maintained in a species, a large number of half-sib families were produced for each of two contrasting populations. These were compared over four nitrogen levels and the ability for selection to operate on phenotypes was evaluated by estimating the narrow sense heritabilities of a number of plant size and phenological traits. It is shown that R. raphanistrum has successfully adapted to local growing environments. Genetic variation between Australian populations of R. raphanistrum was detected in individual growth and developmental traits, as well as integrated phenotypes. Furthermore, variation in flowering time and emergence patterns can be attributed partly to selection operating differently in local habitats. Thus, the genetic diversity within this species is likely to have increased its ability to survive and evolve to a large range of growing conditions. Phenotypic plasticity allows plants from genetically distinct populations to maximise fitness when they encounter small-scale variation. An example of this is the 'shadeavoidance syndrome', which enabled R. raphanistrum plants in this study to compete well and reproduce successfully in environments with high and low competition. While populations showed similar plastic responses in most traits to environmental gradients of nitrogen level and competition, plasticity varied between populations. Genetic variation between populations in plasticity (population x environment interaction) was detected only in traits that were correlated with flowering time. Despite having undergone divergence between populations, significant genetic variation within populations was recorded in all traits measured. Heritabilities for developmental and some growth-related traits were high and varied between environmental levels. Furthermore, genetic variation in plasticity for final plant weight meant that no one genotype was favoured in all of the environmental levels. Thus, combined with outcrossing, dispersal and environmental heterogeneity, phenotypic plasticity is likely to have contributed to the maintenance of genetic variation within populations of R. raphanistrum. Maternally controlled traits also influenced phenotypic variation in R. raphanistrum. Seed size was shown to impact on rosette growth, phenology and final biomass. Differences in integrated phenotypes between plants from large and small seeds were, however, greater in competitive environments and varied between populations. This thesis has thus demonstrated that both phenotypic plasticity and genetic variation can be maintained in a species and that both these factors are likely to have contributed to the success of R. raphanistrum in Australia. Furthermore, given the high levels of genetic variation within populations and the ability of plants to successfully respond to a variety of growing conditions, this species is likely to remain very difficult to manage even if farming systems change considerably.